Hi Anna, if you create a smart list or smart campaign where the smart list has a filter "filled out form", and you might also include not a member of smart campaign [your triggered campaign that they aren't already a member of], then you can action these people.
If you want them to be added to the same campaign that they were missed from, you could add a trigger to the campaign you want them to be added to of "campaign was requested" (you must do this first), and then in the smart campaign that you created above, add a flow step of "request campaign".